1. Technical Field
The present invention relates to integrated circuit amplifier circuitry and, more particularly, to integrated circuit multi-mode drivers.
2. Related Art
Integrated circuit designers often forecast demand for a particular product prior to expending the resources to design, test and develop a product. Because the process for initially producing an integrated circuit product is expensive, designers typically require an expected demand level prior to investing in a new product. In many cases, however, customers have differing environmental requirements for similar products or a particular product may be utilized to operate according to different requirements or standards. Accordingly, design requirements for supply voltages and output signal magnitudes may vary. For example, both 1.5 and 1.0 Volt supplies are common. Similarly, some applications require a peak-to-peak signal magnitude of less than 1.0 Volts while other applications require an output magnitude that exceeds 1.0 Volts. Integrated circuits manufacturers thus often design circuits for multiple applications to avoid having to develop two similar integrated circuits for the different applications.
One common approach is to design a circuit with circuit elements that can withstand the most strenuous applications. A designer of a particular circuit might, for example, choose to use thick oxide devices available in the technology that can withstand the largest voltage magnitudes that may be experienced in the various applications. One problem with this approach, however, is that power consumption is greater for the lower voltage or lower power applications in relation to a device that was designed to operate under lower voltage/power conditions. Stated differently, a thinner oxide device designed for the lower power applications consumes less power and is therefore more efficient than the thicker oxide device in the lower voltage/power applications.
Another common approach is to design multiple circuits into a single device and to modify the system postproduction based on customer requirements. FIGS. 1A-1C illustrate a prior art approach similar to this. Referring to FIG. 1A, a prior art driver system 02 includes a pair of drivers 04 that are coupled by at least one trace. In FIG. 1A, four traces are shown though a different number of traces may readily be used. Drivers 04 have parallel-coupled components. In FIG. 1B, a driver 04 comprises a 1.0 Volt Supply (VDD) coupled to a pair resistors that form a load for driver 04. Each of a pair of MOSFETs is coupled to receive current from each of the resistors of the load. The source terminals of the MOSFETs are commonly coupled to a current source (sink) that in turn is coupled to circuit common. The output nodes of driver 04 (as well as driver 06 of FIG. 1C) are the nodes that couple the drain terminals of the MOSFETs to the resistors. As may be seen here, the output signal magnitude is less than 1.0 Volts and is shown as ranging from 0.5-0.7 Volts peak-to-peak. An input signal applied to the gate terminals of the MOSFETs is amplified and produced at the output terminals as shown. Often, the output signal magnitude is substantially similar but has greater output current capacity to operate as a “driver”.
Referring now to FIG. 1C, it may be seen that the circuit topology of a driver 06 is similar to that of driver 04 of FIG. 1B. Here, however, the supply is 1.5 Volts, the current source produces (sinks) twice the current of driver 04 and the output signal magnitude ranges from 0.8-1.2 Volts peak-to-peak. Additionally, while not shown here, the gate oxide thickness for the MOSFETs of driver 06 is greater than driver 04 to withstand the higher operational voltages for a low swing output. In this case, the current is reduced by half.
In the prior art, designers typically produce two separate drivers for the different applications (e.g., high swing or low swing) or the different supply voltage levels on different cores. Alternatively, as shown in FIG. 1A, two drivers 04 may be fabricated with one or more traces coupling similar circuit elements in parallel to effectively create a driver with the performance capability of driver 06 to operate in KR mode. Thus, post-production, the traces coupling the parallel-coupled components may be cut to permanently create a low swing low supply driver. Thus, one design may be used and subsequently modified post-production according to customer requirements. Such modifications, however, are permanent.